Stair exposes three paintable slots — treads / body / railing (railing only
when railingMode != none) — on node.slots. The 2-material-index body mesh
maps materialIndex 0->treads, 1->body via userData.slotIds; railing meshes
tag userData.slotId='railing'. Per-slot resolution layers over the viewer's
base body/railing materials only in textures-on mode: node.slots ref ->
legacy per-part field (preserved) -> declared default (treads wood-woodplank48,
body wood-woodfine2, railing metal-steel). A custom preview swaps the targeted
body material-array index and whole-mesh railing materials. Monochrome
unchanged. Stops using DEFAULT_STAIR_MATERIAL.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
